The main programming interface to all services within the directory service is XDS/XOM, as defined by X/Open. The calls correspond to the X.500 service requests, including Read, List (enumerate children), Search, Add Entry, Modify Entry, Modify RDN, and Remove Entry. XDS uses XOM to define and manipulate data structures (called objects) used as the parameters to these calls, and used to describe the directory entries manipulated by the calls. XOM is extremely flexible, but also somewhat complex. The interfaces are used in different ways, depending on which underlying directory service is being addressed. For example, CDS entries are typeless, but GDS entries are typed. This difference is reflected in the use of the interface.